1. Field of the Invention
The present invention relates to a transfer device operative to transfer a toner image formed on an image carrier by electrophotographic image formation to a recording medium such as a recording sheet (hereinafter will be referred to as “recording sheet”) via an endless belt. More particularly, the invention relates to a transfer device operative to transfer such a toner image from the image carrier to the endless belt or a recording sheet on the endless belt by the use of a transfer roller.
2. Description of the Related Art
Full-color image forming apparatus perform image formation using color toners corresponding to plural colors which are indicated by respective image data items obtained by color separation of a color image. For example, such image formation includes: reading the same color image through filters for three additive primary colors (red, green and blue); creating image data items respectively indicative of at least three subtractive primary colors (cyan, magenta and yellow) from the data thus read; forming visualized images based on the respective image data items using color toners respectively corresponding to the colors indicated by the respective image data items; and superimposing the visualized images of the respective colors one upon another to form a full-color image.
Heretofore, a tandem type full-color image forming apparatus has been proposed which includes a revolvable semiconducting endless belt and a row of image forming sections arranged in the direction of movement of the outer peripheral surface of the endless belt, the image forming sections being configured to individually form visualized images colored-different from each other, and which forms one full-color image during at least one revolution of the endless belt.
Tandem type full-color image forming apparatus described in Japanese Patent Application Laid-Open Nos. H10-039651 and H10-293437 and Japanese Patent No. 2574804 are capable of higher-speed full-color image formation by employing either an intermediate transfer method including superimposing visualized images of respective colors formed at respective image forming sections one upon another on the outer peripheral surface of an endless belt, followed by transfer of the resulting image to a recording sheet or a transfer feed method including sequentially transferring visualized images of respective colors formed at the respective image forming sections to a surface of a recording medium being fed as attracted onto the outer peripheral surface of an endless belt.
Such a conventional tandem type full-color image forming apparatus is configured to form toner images on respective image carriers based on image information items obtained by color separation at the respective image forming sections and then transfer the toner images from the image carriers to the endless belt or a recording sheet being fed by the endless belt via transfer rollers. As shown in FIGS. 1A and 1B, transfer roller 201 abuts against image carrier 203 with endless belt 202 intervening therebetween. The rotating shaft 201A of the transfer roller 201 extends parallel with the rotating shaft 203A of the image carrier 203 while being positioned on normal LV passing through a point of contact PC between the periphery of the image carrier 203 and the direction of movement of the endless belt 201. The transfer roller 201 is in contact with the endless belt 202 at the point of contact PC on the normal LV.
The peripheral surface of the transfer roller 201 is formed of a material which is softer than the surface of the image carrier 203. The transfer roller 201 is biased toward the rotating shaft 203A of the image carrier 203 along the normal LV and hence is pressed against the image carrier 203 across the endless belt 202. By this arrangement the transfer roller 201 and the image carrier 203 define therebetween a transfer nip region N having a predetermined width in the direction of movement of the endless belt 202. A toner image is transferred from the image carrier 203 to the endless belt 202 or a recording sheet through this transfer nip region N.
Since such a conventional full-color image forming apparatus has the above-described arrangement wherein the transfer nip region having the predetermined width in the direction of movement of the endless belt is defined by pressing the transfer roller having a lower surface hardness than the image carrier against the image carrier, exact transfer of a toner image becomes difficult due to instability in the circumferential velocity ratio between the endless belt and the image carrier which occurs when the velocity of the endless belt passing through the transfer nip region varies. Variations in the velocity of the endless belt passing through the transfer nip region are likely to occur due to variations in the coefficient of friction between the endless belt and the image carrier with environmental changes or with time.
The intermediate transfer type full-color image forming apparatus needs to have an increased nip width defined between the transfer roller and the image carrier in order to ensure reliable transfer of a toner image from the image carrier to the endless belt. As the nip width increases, the endless belt and the image carrier press against each other with increasing force, causing toner to aggregate. Such an aggregate of toner is likely to remain on the endless belt without transfer to a recording sheet. As a result, the image on the recording sheet suffers from voids (the phenomenon that toner forming inside portions of an image such as a character fail to transfer) and, hence, the image quality thereof is degraded. Further, the increase in the pressing force between the endless belt and the image carrier is likely to cause toner present on the upstream side of the endless belt to return to an image carrier on the downstream side, thus raising the problem of disagreement in image color due to undesirable mixture of color toners.
A feature of the present invention is to provide a transfer device for use in image forming apparatus which defines a transfer nip region having a predetermined width in the direction of movement of an endless belt without pressing a transfer roller against an image carrier across the endless belt, thereby preventing variations in the velocity of the endless belt passing through the transfer nip region, degradation in image quality due to toner aggregation, undesired entry of toner into the developing device of another image forming section, and an increase in toner consumption.